Insomnia is the most prevalent sleep disorder, affecting 6-10% of the U.S. population, and is associated with a number of daytime sequelae. However, little is known about the underlying neurobiological mechanisms that might lead to insomnia. There is preliminary evidence that the stress system may play a major role in the etiology of insomnia. The goal of the present study is to examine whether individuals with insomnia are more reactive to stress than healthy sleepers. We hypothesize that insomniacs will show heightened reactivity to the anticipation of a stressor. The proposed project will include 20 individuals with primary insomnia and 20 age- and gender-matched healthy sleeping controls. After collection of background measures and an adaptation night in the sleep laboratory, subjects will undergo a baseline and stress night. At 30 minutes before their habitual bedtime, upon awakening in the morning, and 30 minutes after waking up, subjects will provide a saliva sample and complete ratings of their current level of stress. On the adaptation and baseline nights no additional procedures will take place. On the stress night a mild electric shock will be administered to the subject after it is first demonstrated on the research personnel. They will then be told that they may receive up to 3 additional electric shocks during the night, although they will not actually receive any more shocks. The primary outcome measure is reactivity of sleep latency on the stress night compared to the baseline night. Secondary outcome measures are: subjective ratings of stress, salivary cortisol and alpha-amylase, sleep quantity and quality, high frequency EEG power, and heart rate variability. The results of this study will begin to determine whether the stress system plays a role in the neurobiology of insomnia. Increasing our understanding of the etiology of insomnia may improve prevention and treatment of the condition and reduce the large public health burden of insomnia.